The combustion of fossil fuels based on raw petroleum poses problems in many respects. The world oil consumption is currently around 3.5 billion tons, about 90% thereof being used as fuels. Automobile fuels, power plant fuels, ship fuels and aircraft fuels form the major part thereof.
However, when the consumption remains constant, the world's oil resources will last for only about another 50 to 100 years, so that there is a high need for more efficient combustion methods.
More efficient combustion methods are also required in order to reduce the emission of CO2 as a product of combustion. In the earth's atmosphere, CO2 reflects the heat radiation emitted from the ground. Therefore, the high emission of CO2 is considered a main cause of the greenhouse effect.
Another problem in the combustion of conventional fuels is the emission of noxious substances, which cannot be eliminated completely even by novel fuel-injection and combustion techniques or fuel additives.
Just in the motorized traffic, the load on the air from noxious substances like nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC) and particulate matter (PM) and precursor substances, which adversely affect the ozone balance, causes great problems. These problems can be solved only partially by modern exhaust gas aftertreatment techniques, such as automobile exhaust catalysts. Thus, although diesel fuel can be combusted more efficiently as compared to other fuels, such combustion results in a considerable formation of particulate matter. For technical reasons, an exhaust gas aftertreatment for the removal of NOx is not employed in diesel vehicles currently.
Also, the load on the atmosphere from noxious substances emitted by aircrafts is an as yet unsolved problem.
For the more efficient use of fossil energy sources, improved combustion techniques, such as improved fuel injection in combustion engines, are being developed. However, improved combustion methods often result in an increased emission of noxious substances. Due to thermodynamic laws, the efficiency of combustion engines is enhanced as the combustion temperature increases. However, an increase of combustion temperature often results in an increased emission of noxious substances, especially NOx.
One possibility for the simultaneous improvement of combustion efficiency and reduction of emission of noxious substances is the use of special fuels, especially fuels which consist of a mixture of aqueous and non-aqueous phases, such as water-in-oil (w/o) emulsions. Such fuels allow an efficient combustion process in spite of comparatively low combustion temperatures.
A central point in the use of these particular fuels is the positive effect of the addition of water on the combustion due to the steam engine effect of the evaporating water. This means that water is transferred from the liquid to the vapor state and thereby joins the combustion gases in driving the piston. Due to the evaporation enthalpy, the evaporation of the water reduces the temperature in the combustion chamber, whereby a reduction of the noxious substances NOx and CO, HC and PM (“particulate matter”, soot) in the exhaust gases is achieved.
The use of emulsions of oil and water in various combustion processes has been tested many times. The main disadvantage of such emulsions is their instability, and moreover, their water content is not variable and very low.
Formulations which are described as kinetically stabilized microemulsions are known. Also, the use of thermodynamically stable microemulsions has been described before. These are non-optimum microemulsions (w/o) consisting of water-swollen micelles existing in the fuel. Thus, they are microemulsions comprising exactly one continuous phase. Therefore, the water content in the fuel microemulsions known so far is rather low and is often not more than 20%. Microemulsions with higher water contents have often high or expensive emulsifier fractions. Further, many formulations include high contents (up to 20%) of alcohols.
Most known water-fuel mixtures have only water-in-oil micelles as a microstructure and are not optimum bicontinuous microemulsions. In many inventions, there is a problem in that little water can be emulsified. In addition, for optimizing the combustion, a technology is needed for adjusting the water content of the mixture at will. If the composition of the known water-fuel mixtures is considered in more detail, they are often not water-fuel emulsions with alcohol additions, but rather alcohol-fuel emulsions with low additions of water. Often, the high fugacity of ethanol causes the additional problem that ethanol, but also other more volatile substances are increasingly driven out of the mixture and into the gas phase.
U.S. Pat. No. 4,744,796 describes water/fuel microemulsions with diesel fuel, gasoline, fuel oil and kerosine as the oil component that are stably one-phase and clear over a maximum broad range of temperatures of from −10° C. to +70° C. while showing a high salt tolerance. Their content of the aqueous component consisting of water and/or methanol is from 3 to 40%. As a cosurfactant, tert-butyl alcohol (TBA, 1-20%, with methanol up to 30%) is added to one or more cationic, anionic, amphoteric and non-ionic surfactants (2-20%). Betains with different carbon chain lengths (11-17) are employed as amphoteric surfactants, and ethoxylated alcohols (CiEj), alkylphenols and carboxylates are employed as non-ionic surfactants. Quaternary ammonium salts are used as cationic surfactants, and fatty acids are used as anionic surfactants. These water/fuel microemulsions are non-bicontinuous o/w microemulsions which are not optimal for this purpose. Further, TBA is used as an obligatory cosurfactant in this patent.
U.S. Pat. No. 4,158,551 describes an emulsion of gasoline, water and non-ionic surfactants in order to minimize exhaust gases harmful to the environment in the combustion. The mixture includes up to 22% of water and is stabilized by 1-3.5% of surfactants. The surfactants are essentially ethoxylated alkylphenols with 1.5-30 moles of ethylene oxide per mole of nonylphenol. However, such an emulsion is thermodynamically unstable.
U.S. Pat. No. 6,302,929 describes high-water fuels which are based on two-phase water-continuous (o/w) emulsion systems in contrast to most other known emulsions. As compared to pure hydrocarbons, these fuels have the advantage that they are not inflammable outside the combustion chamber. In the mixtures described, from 20 to 80% of water can be emulsified. Further, the emulsions contain from 2 to 20% of alcohols, low amounts (0.3-1%) on non-ionic surfactants (CiEj, alkylglucosides, Igepal CO-630), and minor proportions of polyorganosiloxanes. The fuel component is gasoline, kerosine, diesel fuel, synthetic and biological fuels, which can be combusted more effectively than the pure hydrocarbons. The high proportion of water decreases the combustion temperature so much that the emission of noxious substances is reduced (CO: −50%). However, the described preparation of the mixtures is difficult to perform, and the combustion composition probably varies widely in the application. In addition, in practice, the engines must be modified to a higher extent for two-phase mixtures (“rotary engines”) as compared to one-phase mixtures.
EP 0 475 620 describes non-temperature-sensitive diesel fuel, gasoline and kerosine microemulsions and their low-pollutant combustion. The mixtures include up to 30% of water, which can be replaced partially or wholly by methanol, ethanol or propanol. In addition to a wide variety of additives (for example, ammonium nitrites, nitrates and halogenates as well as halogen acids and organic compounds) for improving the combustion parameters, a comprehensive selection of emulsifier systems is described which are employed as combinations of at least two different surfactants. Thus, in addition to many ionic surfactants (C8-C30 chains with and without branching/ring) with different head groups (including alkali metals, —SO3H, —NH3 and alkylated, alkanoylated, ethoxylated or sulfonated ammonium), a number of non-ionic surfactants (for example, CiEj, Igepals, ethoxylated alkylphenols) are also used. The classification is not by ionic and non-ionic, but by hydrophilic and lipophilic surfactants (phase condition 2 or 2 at T=20° C., Φ=α=0.5 and γ=0.02). In addition, a wide range of cosurfactants (medium-chain alcohols, glycol ethers and ethers) are employed. What is described is one-phase, transparent microemulsions. However, one-phase microemulsions with 2% of surfactant are optically turbid, and therefore, it can be assumed that the optically clear microemulsions must contain more than 10% of surfactant. Such mixtures with a low water/surfactant ratio are not sufficiently efficient for an economical application.
U.S. Pat. No. 5,669,938 describes one-phase w/o emulsions consisting of diesel fuel and 1-40% of water and surfactant for the reduction of noxious substances (CO, NOx, HC, PM, soot). A key characteristic is the use of organic alkyl nitrates. Linear hydrocarbons having a chain length of from 5 to 10 carbon atoms as well as branched hydrocarbons, especially the 2-ethylhexyl residue, serve as alkyl residues.
U.S. Pat. No. 4,451,265 describes one-phase, clear fuel/water microemolsions which have high stabilities at low temperatures. In their non-elucidated microstructure, the existence of w/o micelles is assumed. The mixtures consist of diesel fuel (34-99%), water (0.1-6%), alcohol (0.5-42%) and a surfactant system (0.5-58%). As alcohols, which make up the vast majority of the aqueous phase (Ψeth=70-95%), mainly ethanol, but also methanol and propanol are used. The water proportion in the emulsion is limited to a maximum of 6%. Described are microemulsions with technical surfactants which have a hydrophilic N,N-dimethyl-ethanolamine head and a hydrophobic fatty acid residue with a carbon chain length of from 9 to 22 atoms, especially fatty acids from soybean.
U.S. Pat. No. 4,451,267 describes microemulsion fuels made from vegetable oils. Mainly soybean oil, but also many other oils, for example, rapeseed oil, are used as vegetable oils. The aqueous component of the low-water microemulsions mainly consists of methanol, ethanol or propanol (Ψeth=70-95%). Amines trialkylated with long-chain fatty acids are used as surfactants and supplemented by large amounts of butanol as a cosurfactant (about 20%). In this case too, w/o micelles are assumed as microstructures.
U.S. Pat. No. 4,002,435 describes w/o emulsions with gasoline which are stably mono-phase over a wide range of temperatures and are based on large proportions of alcohol (0.1-20%). As alcohols, methanol, ethanol and isopropanol are employed. The emulsions contain little water (0.1-10%) and a mixture or organic oleate, linolate and stearate salts, oleic acid as well as phenolated and ethoxylated fatty alcohols.
U.S. Pat. No. 4,599,088 describes gasoline emulsion fuels with 2-10% of alcohol, such as methanol, ethanol, isopropanol or TBA. However, the formulations contain only 0.1-0.5% of water. The mixtures include 0.1-3.0% of surfactants, which are exclusively non-ionic alkylphenols and CiEj surfactants, where i=9-24, and j=6-10. The mixtures are referred to as w/o type one-phase microemulsions (micelles). However, little water can be dissolved therein. Higher additions of water result in a water excess phase in the fuel tank.
U.S. Pat. No. 5,104,418 describes microemulsion systems of water, diesel fuel, glycolipid (surfactant) and aliphatic alcohols (cosurfactant). The microemulsions are stably monophase from 0° C. to 80° C. The description comprises glycolipids of the form A-X-R, where the hydrophilic surfactant heads A may be glucose, mono-, di-, tri- and tetrasaccharides. As the hydrophobic residues R, saturated, mono- and polyunsaturated, linear and branched hydrocarbon chains having a carbon chain length of from 10 to 24 atoms are mentioned; they are linked with the surfactant head through the functional groups X=ether, ester, acetal and hemiacetal. The microemulsions are defined as a thermodynamically stable colloidal dispersion. In this case too, for large diesel fuel contents (60-90%), the water contents are very low, being 1-10%. In contrast, the cosurfactant content (Butanol, pentanol, hexanol) is very high, being 6.3-21%, and the glycolipid content is 1.7-9%. U.S. Pat. No. 5,259,851 describes similar water/fuel/glycolipid/cosurfactant microemulsions with the same glycolipids and similar mixing ratios. However, different cosurfactants are employed here, namely aliphatic diols, and in addition to diesel fuel, there are also employed gasoline, fuel oil, kerosine and other oils.
U.S. Pat. No. 4,465,494 and EP 0 058 605 describe microemulsions of water, fuel (including fuel oil), surfactant and additive (special alcohols and amines), which are stably monophase from −20° C. to +100° C. (in part only from −10° C. to +20° C.). In addition to 1-27% of alcohol (methanol, ethanol, isobutanol and ethyl-2-hexanol), these mixtures include only 1-10% of water. Benzyl amines and phenoxyalkylated organic acid salts (counter ion: metal ion or organic base) of different carbon chain lengths are employed as surfactants. The microemulsions are efficient with a surfactant content of 1-10%. Further, in addition to a method for the preparation of the microemulsions, the reduction of emissions during their combustion is described. The emission of CO is reduced by 80%, and that of NOx by 75%, based on 100 kilometers driven, as compared to conventional fuels.
U.S. Pat. No. 6,017,368 describes microemulsions which contain water, fuel, anionic and non-ionic surfactants, unsaturated fatty acids, aliphatic alcohols and ethanol or methanol. They are water-in-oil micelles with a low water content of from 1 to 10%. These microemulsions are stable over a wide range of temperatures, have a low viscosity and have a reduced emission of noxious substances during combustion. Gasoline and fuel oil are employed as fuels in addition to diesel fuel. The content of water-soluble alcohols, being from 6 to 14%, is higher than the water content. The water-insoluble alcohols (from 1 to 10%) have a hydrocarbon chain length of from 5 to 9 atoms. The anionic surfactants employed (2 to 10%) are based on ammonium-neutralized unsaturated fatty acids, for example, from soybean oil. As non-ionic surfactants (1 to 5%), non-ethoxylated compounds are exclusively employed because ethoxylated compounds have poor combustion properties according to U.S. Pat. No. 6,017,368. Only 2,4,7,9-tetramethyl-5-decyne-4,7-diol is mentioned as a non-ionic surfactant.
EP 1 101 815 describes diesel fuel/water microemulsions which contain an emulsifier and an emulsifiable agent, especially sorbitan monooleate and nonylphenol ethoxylate. However, the water content is limited to a narrow range of concentrations (100-145 parts of water, based on 1000 parts of diesel fuel).
WO 00/31216 and EP 1 137 743 describe a diesel fuel composition consisting of diesel fuel, (water-containing) ethanol, a polymeric stabilization additive and optionally an alkyl ester of a fatty acid and/or an auxiliary solvent, such as s short-chain alkyl alcohol. However, the water content of the ethanol employed is at most 5% by weight, based on the amount of ethanol in the mixture.
DE 10003105, WO 01/55282 and EP 1 252 272 describe fuel-water emulsions in which an alkoxylated polyisobutene is used as the emulsifier. The emulsion preferably contains 10-25% of water and 0.2-10% by weight of emulsifier.
The water content in the water/fuel microemulsions known so far is low. It is often not more than 5 to 20%, less frequently up to 40%. Water/fuel microemulsions with higher water proportions can be found in very few descriptions, and with uneconomically high emulsifier contents. Further, many formulations include high contents (up to 20%) of alcohols (methanol, ethanol and in part also longer-chain alcohols).
Disadvantages of the described emulsions and methods are their low emulsion stability, the high content of emulsifier, which is cost-intensive, or an insufficient systematic knowledge on the phase behavior as well as mechanisms during combustion. However, these are a precondition for designing an optimum formulation for an optimum combustion.
Conventional water/fuel mixtures have water-in-oil micelles as microstructures and are not optimum bicontinuous microemulsions. Due to this fact, there is often a problem in that little water can be emulsified. For optimizing the combustion, the technology for freely adjusting the water content of the mixture is often lacking.
Some conventional water/fuel mixtures are not water/fuel emulsions with alcohol additions, but only alcohol/fuel emulsions with low additions of water. The high fugacity of ethanol causes the additional problem that ethanol, but also other more volatile substances are increasingly driven out of the mixture and into the gas phase.